hardening process, both pieces will have swelled in volume,
and it will be found that the pin will fit the socket more tightly
than before. The final fit may be obtained by lapping the pin
with emery powder in the lathe.
1 P. 128. Hardening Steel.—The difficulty experienced in
hardening milling cutters without cracking is found to be largely
due to unequal heating as well as unequal cooling. To avoid the
former a method of heating in a bath of molten lead kept at a
high temperature is found to be very successful. Regular cooling
is very difficult to obtain in the case of thin articles, such as
circular saws; but by placing a sheet of brown paper upon the
surface of oil and allowing the article, placed upon the paper,
to gradually sink into the liquid, warping may be largely pre-
vented, though nothing softer than the equivalent of a brown
colour can be thus obtained. If a saw is to be tempered* to blue,
the usual course of water tempering must be followed, dipping
as smartly as possible, and the blade be straightened afterwards.
Hardening in water and tempering subsequently in oil will pro-
duce a softer result than if water be used throughout.
P. 152. Lathe Centreing.—By adopting a very slightly
more acute apex angle for the centreing drill than for the lathe
•centre, the necessity for drilling the small hole, as mentioned at
top of p. 152, is avoided. See Fig. 733.
P- *53> Double Driving.—By allowing both ends of the
•carrier to be driven from the catch plate, stress is taken off the
lathe centre, and more steady tooling is produced. Clements'
driver, Fig. 734, is designed to effect this purpose. The carrier c